GW1 = Works as a router, automatically picking the "best" connection between Inet1 and Inet2 (the latter via GW2).

GW2 = 5GHz wifi router

And here's the problem:
H3 only needs internet access when Inet2 is up. What i was thinking of doing was a routing table that looks like this:

route to GW2 via GW1

default route is via GW2

I first set the route to GW2 via GW1 without a problem. But when i try

route add default gw 1.2.3.4

(1.2.3.4 being the IP of GW2), it complains "SIOCADDRT: No such device"
Is the problem that the default gw i'm trying to set is not reachable directly? Is there a different approach that would allow me to achieve this?

An alternative (and hypothetical) approach: Since H3 will be using a static IP, is it possible to do some magic with iptables on GW1 to forward any packets from H3 to GW3, thereby "tricking" H3 into using GW2 as its default router?

@ChristopherPerrin proprietary software that has multiple wan tunnels going through a collection of 3g modems and CDMA modems. It'd be easiest to just use iptables or something to have any packets avoid that mess alltogether.
–
JarmundOct 7 '12 at 23:32

I don't have the possibility to test the commands, but they should be correct. Routing rules in table 999 are not visible in normal $ ip route show - you need to append the table id: $ ip route show table 999.

You can of course choose whatever table number you like. It's also possible to use names. First assign an id to a name: $ echo <id> <name> /etc/iproute2/rt_tables. Then you can use commands like $ ip route show table <name> and $ ip rule add ... table <name>.
–
porOct 8 '12 at 2:43

Since your question is light on specifics, the answer will also be light on specifics:

Set H3's default gateway to GW1.

Have GW1 always route traffic from H3 to GW2.

The "intelligence" of how to route packets must reside in the routers, not the end boxes. So your default gateway must be one directly accessible on the same subnet; that gateway must then figure out what to do with the packets to get them wherever they should go.